1. Field of the Invention
The present invention relates to tantalum powder and the preparation of the same, and also relates to the electrolytic capacitor anode made of the tantalum powder. More particularly, the present invention relates to the improved powder for manufacturing sintered tantalum capacitor anode, and the method for preparing the same.
2. Description of the Relevant Art
Tantalum powder is usually prepared by means of making a tantalum metal compound reacting with a reducing agent, or hydrogenating and pulverizing the dense metal. Generally, the tantalum powder obtained by means of the former method is porous basic agglomerates having high specific surface area composed of many primary particles, and the tantalum powder obtained by the later method is of high purity, but with low specific surface area.
Typically, sintered tantalum capacitor is made as following steps: compressing tantalum powder to form a pellet, sintering the pellet in a vacuum furnace to form a porous body, subsequently, anodizing the porous body in suitable electrolyte liquid, such as diluted phosphoric acid, to form continuous dielectric oxide film on the surface of the porous sintered body.
Generally, it is desired that an electrolytic capacitor shall have performance characteristics of higher specific capacitance, lower DC leakage and higher breakdown voltage. Since electrolytic capacitor has a lot of useful properties, it has been used in a wide ranging field.
The tantalum powder for manufacturing electrolytic capacitor for high voltage field must have good electric performance, such as higher capacitance and higher breakdown voltage. Usually, for same type of tantalum powder, the larger the specific surface area of the tantalum powder is, the more complex of the particles of the powder are, and the lower the breakdown voltage is. The tantalum powder having poor tolerance of high voltage can hardly be used for high voltage field. The BET surface area of tantalum powder produced by reduction of potassium fluorotantalate with sodium is 0.2˜6.0 m2/g.
In the high voltage tantalum powder field, many researchers have made great efforts for improving the agglomerating performance of tantalum powder, for increasing the capacitance and breakdown voltage of tantalum powder. WO 93/03,191, U.S. Pat. Nos. 4,740,238, 5,211,741, 5,261,942 disclosed improved flake tantalum powder and processes for producing flake tantalum powder, but the processes are complex, difficult to control, and the cost is high, thus the application of these technologies is limited.
Accordingly, there is a need in the prior art for the tantalum powder that can be used in high voltage electrolytic capacitor. Particularly, it is desired that the tantalum powder have high capacitance, excellent performance on DC leakage and good voltage tolerance. Furthermore, it is needed a simple, low cost and easily-controlled process in which tantalum powder with the excellent performance and having high specific capacitance and good tolerance of high voltage can be prepared.